Precipitation nowcasting during the Vancouver 2010 Olympics: The impact of Diabatic Cooling of Melting Snow on Precipitation Phase and Intensity

Nowcasts of preciptation intensity, duration and phase were critical information needed by many of the Olympic venues managers during the Vancouver Olympics. The diabatic cooling effect of melting snow was found to play an important role in this nowcast. This paper will examine one case of diabatic cooling in the Whistler/Callaghan venue area that was well documented with surface, radar, radiometer,and upper air observations. The case occurred on February 13, 2010. A rapid cooling was observed at instrument sites experiencing precipitation after about 2200 UTC. Near the base of Whistler Mountain the temperature cooled from 2 to 0 ˚C. This led to the creation of a nearly isothermal layer, shift of the valley flow from up-valley to down-valley flow towards Squamish (located to the south of Whistler), and the continuation of snow for 4-5 hours despite the expectation of large scale warm advection to maintain temperatures above freezing. The precipitation-induced down-valley flow may act like a secondary cold front that further enhances and prolongs the precipitation. The nowcasting challenge was to determine whether the diabatic cooling would counter the warm advection and for how long. This case study provided a well observed situation to examine the competing roles of warm air advection versus diabatic cooling of melting snow on precipitation phase in a region of complex terrain. We will also present the 1 km LAM real-time model forecasts of this case.